Forgive my ignorance, but what is "Z/2"? On Tue, Jun 27, 2017 at 5:35 PM, Nathaniel Smith <n...@pobox.com> wrote:
> On Jun 26, 2017 6:56 PM, "Charles R Harris" <charlesr.har...@gmail.com> > wrote: > > >> On 27 Jun 2017, 9:25 AM +1000, Nathaniel Smith <n...@pobox.com>, wrote: >> > I guess my preference would be: >> 1) deprecate + >> 2) move binary - back to deprecated-but-not-an-error >> 3) fix np.diff to use logical_xor when the inputs are boolean, since >> that seems to be what people expect >> 4) keep unary - as an error >> >> And if we want to be less aggressive, then a reasonable alternative would >> be: >> 1) deprecate + >> 2) un-deprecate binary - >> 3) keep unary - as an error >> >> > Using '+' for 'or' and '*' for 'and' is pretty common and the variation of > '+' for 'xor' was common back in the day because 'and' and 'xor' make > boolean algebra a ring, which appealed to mathematicians as opposed to > everyone else ;) > > > '+' for 'xor' and '*' for 'and' is perfectly natural; that's just + and * > in Z/2. It's not only a ring, it's a field! '+' for 'or' is much weirder; > why would you use '+' for an operation that's not even invertible? I guess > it's a semi-ring. But we have the '|' character right there; there's no > expectation that every weird mathematical notation will be matched in > numpy... The most notable is that '*' doesn't mean matrix multiplication. > > > You can see the same progression in measure theory where eventually > intersection and xor (symmetric difference) was replaced with union and > complement. Using '-' for xor is something I hadn't seen outside of numpy, > but I suspect it must be standard somewhere. I would leave '*' and '+' > alone, as the breakage and inconvenience from removing them would be > significant. > > > '*' doesn't bother me, because it really does have only one sensible > behavior; even built-in bool() effectively uses 'and' for '*'. > > But, now I remember... The major issue here is that some people want > dot(a, b) on Boolean matrices to use these semantics, right? Because in > this particular case it leads to some useful connections to the matrix > representation for logical relations [1]. So it's sort of similar to the > diff() case. For the basic operation, using '|' or '^' is fine, but there > are these derived operations like 'dot' and 'diff' where people have > different expectations. > > I guess Juan's example of 'sum' is relevant here too. It's pretty weird > that if 'a' and 'b' are one-dimensional boolean arrays, 'a @ b' and 'sum(a > * b)' give totally different results. > > So that's the fundamental problem: there are a ton of possible conventions > that are each appealing in one narrow context, and they all contradict each > other, so trying to shove them all into numpy simultaneously is messy. > > I'm glad we at least seem to have succeeded in getting rid of unary '-', > that one was particularly indefensible in the context of everything else > :-). For the rest, I'm really not sure whether it's better to deprecate > everything and tell people to use specialized tools for specialized > purposes (e.g. add a 'logical_dot'), or to special case the high-level > operations people want (make 'dot' and 'diff' continue to work, but > deprecate + and -), or just leave the whole incoherent mish-mash alone. > > -n > > [1] https://en.wikipedia.org/wiki/Logical_matrix > > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@python.org > https://mail.python.org/mailman/listinfo/numpy-discussion > >
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